Image forming apparatus and image forming method for correcting registration deviation

ABSTRACT

There exists a problem that, for registration correction, carrying out less-than-one-pixel-basis correction and one-pixel-basis correction for an image after half toning using screen processing causes unevenness in density when it is hard for linearity of image density against a PWM to come out. Processing of interpolation is carried out so that the way how a halfdot is formed may always becomes the same after the interpolation processing. By applying a matrix in consideration of a dot direction which may be broken by the interpolation processing at the time of the screen processing, conversion in which a way of locating the half dot maintains relation of vertical inversion in a subscanning direction is carried out, and less-than-one-pixel-basis correction is carried out.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus forcorrecting registration deviation.

2. Description of the Related Art

An electrophotography is known as an image recording system used in acolor image forming apparatus such as a color printer or a color copyingmachine. The electrophotography is one which forms a latent image on aphotoconductive drum by utilizing a laser beam, and develops it with acharged color material (hereinafter referred to as toner). Recording ofan image is carried out by transferring and fixing the image developedusing the toner onto a transfer paper.

Recent years, because of image formation speed-up of theelectrophotographic color image forming apparatus, increasing are tandemtype color image forming apparatuses having the same number ofdeveloping machines and photoconductive drums as a number of tonercolors, and transferring images of different colors one after another onan image transfer belt, and on a recording medium. In this tandem typecolor image forming apparatus, it is known that a plurality of factorscausing the registration deviation exist, and various solutions areproposed for each of the factors.

One of the factors is nonuniformity or fixing-position deviation of lensof a deflection scanner, and attaching-position deviation of thedeflection scanner against the main body of the color image formingapparatus. Because of this position deviation, an inclination and acurvature arise in the scanning line, and a degree of the curvature(hereinafter referred to as a profile) becomes different for everycolor, which leads to the registration deviation. The profile differs incharacteristic thereof in each image forming apparatus, that is, inevery recording engine, furthermore in each color.

As a countermeasure method for this registration deviation, there existsa method of measuring magnitude of the inclination and the curvature ofthe scanning line by using an optical sensor, correcting bitmap imagedata so that they may be canceled, and forming the corrected image (forexample, refer to Japanese Patent Laid-Open No. 2004-170755). In thismethod, since the correction is carried out electrically by processingthe image data, a mechanical adjustment member and an adjustment processat the time of assembly becomes unnecessary. Therefore, it becomespossible to miniaturize magnitude of the color image forming apparatus,and the registration deviation can be coped with at low cost. Thiselectric registration deviation correction is divided intoone-pixel-basis correction and less-than-one-pixel-basis-basiscorrection. The one-pixel-basis correction makes the pixel offset in asubscanning direction by one-pixel-basis depending on the correctionamount of the inclination and curvature. In the case of using thismethod, in the image forming apparatus in which the curvature andinclination are about several 100 to 500 micrometers, and the resolutionis 600 dpi, an image memory for several ten lines is needed in order tocarryout the correction. In the following descriptions, a position madeto be offset is referred to as a change point.

The less-than-one-pixel-basis correction adjusts a tone value of imagedata by before and after pixels in the subscanning direction, asillustrated in FIG. 23. That is, when it has bent upward by the profile,the image data before correction is corrected to a reverse direction inthe subscanning side against the direction which the profile indicates.With such a method, by carrying out the less-than-one-pixel-basiscorrection, an unnatural level difference at the change point boundaryline arisen by the one-pixel-basis correction can be dissolved, andsmoothing of the image can be attained.

In the case of carrying out the smoothing processing for the imagehaving progressed to a screen processing immediately before printing,the smoothing is carried out by carrying out pulse width modulation(PWM: Pulse Width Modulation) for the laser beam and switching the laserexposure time gradually in the subscanning direction. For example, inthe case of a 0.5-pixel less-than-one-pixel-basis correction, a halfexposure will be carried out twice vertically in the subscanningdirection. Such interpolation processing is materialized for the firsttime in the relation that an image density is linear against theexposure time of the PWM, and there are many cases where the samedensity as one of the exposure for one pixel can not reproduced by twoof the exposures for 0.5 pixels even under any kinds of conditions.There also exists one which, also against such problems, in addition tointerpolating calculation, devises the way to make lighting-up of thePWM get up right and left, etc., and makes an image centroid shift, andcarries out registration correction. (Japanese Patent Laid-Open No.2001-309139) However, when the linearity of the density against the PWMcannot be held, the image data where it is more preferable to carry outthe above-mentioned interpolation processing and the image data whereimage quality thereof will be impaired if the correction is carried outwill exist mixedly.

For example, as for a repeated pattern of the same pattern or a design(hereinafter, referred to as pattern image), and as for a character anda thin line or the like which are able to be rendered with officedocument creation software, visibility of information thereof isenhanced by carrying out the interpolation processing and smoothing. Onthe contrary, when the interpolation processing is carried out in thechange point of a continuous tone image for which the screen processinghas been carried out, there exists a problem that unevenness in densityarises only on the change point, and the image quality degrades. As thereason, for example in the case of a line growth screen being used,since a thickness of a line which composes the screen will have changedon the change point due to the interpolation processing, macroscopicallyit seems that the density has changed. When the interpolation processingis carried out against add-on images such as tint blocks, the effect maybe impaired, and therefore, the interpolation processing is notsuitable.

As described above, in a situation where it is hard to make linearity ofthe density against the PWM pulse width come out, it is not appropriateto carry out the interpolation processing for any kinds of images, and acertain decision processing has become needed. However, even in the caseof carrying out the decision processing, an erroneous decision mayarise, and in the case of carrying out high-speed decision processing,there exist problems in which a circuit becomes complicated or the like.As a result of the decision processing, when the interpolationprocessing is not carried out for the continuous tone image for whichthe screen processing has been carried out, the level difference of onepixel which occurs at the change point will be allowed, and the leveldifference also becomes recognized visually as the degradation dependingon a type of the image.

Then, for the image for which the screen processing has been carriedout, the correction processing which does not make unevenness in densityor the level difference of one pixel which may occur on the change pointarise even in a situation where it is hard to make the linearity comeout easily, has been required.

SUMMARY OF THE INVENTION

An image forming apparatus of the present invention comprises aninterpolation processing component configured to carry outless-than-one-pixel-basis correction of image data by correcting densityof each pixel of the image data so that sequence of pulse widthaccording to the density of the pixel in a subscanning direction maycome into inverted relationship before and after a change point that isa location where one-pixel-basis correction is carried out for the imagedata, and an image forming component configured to form an image basedon a signal acquired by carrying out a pulse width modulation accordingto the corrected density of the image data.

According to the invention of the present application, in the case ofcorrecting image failures accompanying registration deviation usingdigital image processing, it become possible to carry out satisfactorycorrection without making unevenness in density or the level differenceof one pixel which occurs at the change point arise also in the locationwhere the screen processing has been carried out.

Then, at that time, the PWM is used for the laser beam, and also for onein which the linearity of the density against the pulse width cannotcome out easily, carrying out the correction satisfactorily is realized.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an imageforming apparatus;

FIG. 2 is a sectional view of the image forming apparatus;

FIGS. 3A and 3B are figures illustrating an example of a profilecharacteristic of the image forming apparatus;

FIGS. 4A to 4D are figures illustrating a relation between a deviationin the image forming apparatus and a correction direction thereof;

FIGS. 5A to 5C are figures illustrating a data holding method of theprofile characteristic;

FIG. 6 is a block diagram illustrating a configuration of a HTprocessing part in Embodiment 1;

FIG. 7 is a figure illustrating an example of a change point and aninterpolation processing region;

FIG. 8 is a figure illustrating schematically processing according tothe conventional change point of a pixel;

FIG. 9 is a figure illustrating schematically an aspect of pulses of aPWM value;

FIGS. 10A to 10C are figures illustrating schematically an exposureimage against a PWM value;

FIG. 11 is a figure illustrating schematically the processing accordingto change of a pixel in Embodiment 1;

FIGS. 12A to 12C are figures illustrating schematically a data statuswhich a memory part holds;

FIG. 13 is a figure illustrating schematically an aspect of centroidposition movement of a dot;

FIG. 14 is a figure illustrating a principle of screen processing by adither method;

FIGS. 15A and 15B are figures illustrating schematically an aspect of aninput/output of an image by the dither method;

FIG. 16 is an example of an image data where the change is carried outfor the screen image;

FIG. 17 is a block diagram illustrating a detailed configuration of theHT processing part in Embodiment 2;

FIG. 18 is a figure illustrating schematically processing according tochange of a pixel in Embodiment 2;

FIG. 19 is an example of the image data where the change is carried outfor a screen image in Embodiment 2;

FIG. 20 is a block diagram illustrating a detailed configuration of theHT processing part in Embodiment 3;

FIG. 21 is an example of an input/output of an up-down-center flag of adot in Embodiment 3;

FIG. 22 is an example of an image data where the change is carried outagainst a screen image in Embodiment 3; and

FIG. 23 is a figure illustrating an example of correction processingless-than-one-pixel-basis.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the best mode for carrying out the present invention willbe described using drawings.

Embodiment 1

FIG. 1 shows a figure illustrating a configuration of each blockassociated with electrostatic latent image creation in anelectrophotographic color image forming apparatus in the presentembodiment. The color image forming apparatus is made up of an imageformation part 101 and an image processing part 102, and creates bitmapimage information in the image processing part 102, and based on thecreated bitmap image information, the image formation part 101 carriesout image formation onto a recording-medium. FIG. 2 shows a sectionalview of the color image forming apparatus using a tandem typeelectrophotography with an intermediate transfer body 28 adopted. Byusing FIG. 1, operation of the image formation part 101 in theelectrophotographic color image forming apparatus will be described.

The image formation part 101 drives an exposing light depending on anexposure time which the image processing part 102 has processed, formsthe electrostatic latent image, and develops this electrostatic latentimage to form a monochrome toner image. It superimposes this monochrometoner image to form a multicolor toner image, and transfers thismulticolor toner image on the recording medium 11 in FIG. 2 to fix themulticolor toner image on the recording medium.

Reference numerals 23Y, 23M, 23C and 23K in FIG. 2 denote injectioncharging devices, and in order to make photo conductors 22Y, 22M, 22Cand 22K charged for every color of Y, M, C and K, there exist four sets.

Each injection charging device is provided with sleeves 23YS, 23MS, 23CSand 23KS.

Photo conductors 22Y, 22M, 22C and 22K are ones which rotate with adriving force of a not shown drive motor transferred, and the drivemotor makes the photo conductors 22Y, 22M, 22C and 22K rotate in ananticlockwise direction depending on the image formation operation. Anexposure component is configured to form the electrostatic latent imageby irradiating the exposing light to the photo conductors 22Y, 22M, 22Cand 22K from scanner parts 24Y, 24M, 24C and 24K, and exposing surfacesof the photo conductors 22Y, 22M, 22C and 22K selectively.

As for developing devices 26Y, 26M, 26C and 26K in FIG. 2, in order tovisualize the electrostatic latent image, there exist four units ofdeveloping devices which carry out the development for every color of Y,M, C and K, and in each of developing devices, sleeves 26YS, 26MS, 26CSand 26KS are provided. In addition, each developing device 26 isdetachable.

The intermediate transfer body 28 in FIG. 2 rotates in clockwisedirection in order to receive the monochrome toner image from the photoconductor 22, and as the photo conductors 22Y, 22M, 22C and 22K, andprimary transfer rollers 27Y, 27M, 27C and 27K located at the opposedposition rotate, the monochrome toner image is transferred thereon. Byapplying a suitable bias voltage to the primary transfer roller 27 andby differentiating a revolution speed of the photo conductor 22 and therevolution speed of the intermediate transfer body 28, the monochrometoner image is efficiently transferred on the intermediate transfer body28. This is called primary transfer.

Furthermore, the monochrome toner image of every station is superimposedon the intermediate transfer body 28. The superimposed multicolor tonerimage is transferred to a secondary transfer roller 29 as theintermediate transfer body 28 rotates. At the same time, the recordingmedium 11 is conveyed in a sandwiched state from a medium tray 21 to thesecondary transfer roller 29, and the multicolor toner image on theintermediate transfer body 28 is transferred on the recording medium 11.At this time, by applying a suitable bias voltage to the secondarytransfer roller 29, the toner image is electrostatically transferred.This is called a secondary transfer. The secondary transfer roller 29,while transferring the multicolor toner image on the recording medium11, abuts on the recording medium 11 at the position 29 a, and afterprinting processing, is spaced apart to the position 29 b.

A fixing device 31, in order to make the multicolor toner imagetransferred on the recording medium 11 to be melted and fixed thereon,is provided with a fixing roller 32 which heats the recording medium 11and with a pressurizing roller 33 for bringing the recording medium 11into a pressure-contact with the fixing roller 32. The fixing roller 32and the pressurizing roller 33 are formed in a hollow shape, and haveheaters 34 and 35 incorporated therein, respectively. The fixing device31 conveys the recording medium 11 holding the multicolor toner image bythe fixing roller 32 and the pressurizing roller 33, and applies heatand pressure to fix the toner on the recording medium 11.

After that, the recording medium 11 after the toner fixing is ejected bya not shown discharge roller to a not shown paper ejection tray, and theimage formation operation is completed. A cleaning component 30 is onewhich cleans the toner remaining on the intermediate transfer body 28,and waste toner remaining after the multicolor toner image of fourcolors formed on the intermediate transfer body 28 has been transferredon the recording medium 11 is stored in a cleaner container.

Subsequently, a profile characteristic of a scanning line for everycolor of the image forming apparatus will be described using FIGS. 3A,3B, 4A to 4D, and 5A to 5C. FIG. 3A, as the profile characteristic ofthe image forming apparatus, shows a figure illustrating a regiondeviated upward (in vertical direction) against a laser scanningdirection. FIG. 3B, as the profile characteristic of the image formingapparatus, shows a figure illustrating a region deviated downwardagainst the laser scanning direction. Reference numeral 301 denotes anideal scanning line, and indicates the characteristic in the case of thescanning being carried out vertically against the rotation direction ofthe photo conductor 22.

Hereinafter, the profile characteristic in the following description,will be described on the premise of the direction in which correctionshould be made in the image processing part 102, and however, thedefinition as the profile characteristic is not limited to this. Thatis, it is made to be defined as a deviation direction of the imageformation part 101, and the image processing part 102 may be made up sothat the correction for the reverse characteristic may be carried out.In FIGS. 4A to 4D, depending on the profile definition, a correlationbetween a figure illustrating the direction on which the correctionshould be made in the image processing part 102 and a figureillustrating the deviation direction of the image formation part 101 isillustrated. As the direction on which the correction should be made inthe image processing part 102, in the case of a curvature characteristicbeing illustrated like FIG. 4A, the profile characteristic of the imageformation part 101 will become one as illustrated in FIG. 4B in whichthe reverse direction is indicated. On the contrary, as the curvaturecharacteristic of the image formation part 101, in the case of FIG. 4Cbeing shown, the direction in which the correction should be made in theimage processing part 102 becomes as illustrated in FIG. 4D.

As a method of holding data of the profile characteristic, for exampleas illustrated in FIGS. 5A to 5C, a pixel position of a change point(change location) in the main scanning direction and directivity of thevariation up to the next change point are arranged in such a way thatthey are held. Specifically, for the profile characteristic illustratedin FIG. 5A, change points P1, P2, P3, and . . . Pm are defined. Thedefinition of each change point is the point where one pixel deviationoccurs in the subscanning direction, and as the direction, there exist acase in which it changes upward and a case in which it changes downward,to the next change point.

For example, the change point P2 becomes the point where the changeshould be made upward to the next change point P3. Therefore, a changedirection in P2 becomes upward (↑) as illustrated in FIG. 5B.

Similarly, also in P3, the change direction becomes upward (↑) up to thenext change point P4. The change direction in the change point P4,unlike the direction up to now, becomes downward (↓) As a method ofholding this direction data, for example, if it is “1” as the dataindicating the upward direction, and “0” as the data indicating thedownward direction, the method becomes as illustrated in FIG. 5C. Inthis case, the number of the data to be held becomes the same as thenumber of the change points, and if the number of the change points ism, the number of bits to be held will also become m.

Reference numeral 302 denotes an actual scanning line which is caused bythe inclination and the curvature resulting from position accuracy anddeviation in a diameter of the photo conductor 22, and the positionaccuracy of an optical system in the scanner part 24 (24C, 24M, 24Y and24K) of each color illustrated in FIG. 2. As for the image formingapparatus, this profile characteristic differs in every recording device(recording engine) thereof, and furthermore, in the case of the colorimage forming apparatus, the characteristic differs for every color.

Subsequently, using FIG. 3A, the change point in the region where thelaser scanning direction is deviated upward will be described.

The change point in the present embodiment denotes the point which isdeviated by one pixel in the subscanning direction. That is, in FIG. 3A,the points P1, P2, and P3 which are deviated by one pixel in thesubscanning direction on the upward curvature characteristic 302correspond to the change points. FIG. 3A is illustrated with P0 as thebase. As understood also from the same figure, a distance (L1, L2)between two change points becomes short in the region where thecurvature characteristic 302 changes abruptly, and becomes long in theregion where the characteristic changes gradually.

Subsequently, using FIG. 3B, the change point in the region where thelaser scanning direction is deviated downward will be described. Also inthe region indicating the characteristic deviated downward, thedefinition of the change point denotes the point which is deviated byone pixel in the subscanning direction. That is, in FIG. 3B, the pointsPn and Pn+1 which are deviated by one pixel in the subscanning directionon the downward curvature characteristic 302 correspond to the changepoints.

Also in FIG. 3B, like FIG. 3A, the distance (Ln, Ln+1) between twochange points becomes short in the region where the curvaturecharacteristic 302 changes abruptly, and becomes long in the regionwhere the characteristic changes gradually.

Thus, the change point is closely related to a change degree of thecurvature characteristic 302 which the image forming apparatus has.Therefore, in the image forming apparatus with an abrupt curvaturecharacteristic, the number of change points increases, and on thecontrary, in the image forming apparatus with a gradual curvaturecharacteristic, the number of change points decreases.

As already described, since the curvature characteristic which the imageforming apparatus has is different also for every color, the number andposition of the change points thereof differ from each other. Thedifferences resulting from colors will cause the registration deviationto appear in the image where the toner image of all the colors has beentransferred on the intermediate transfer body 28.

Subsequently, using FIG. 1, processing of the image processing part 102in the color image forming apparatus will be described. An imagegeneration part 104 creates raster image data of which print processingis possible from print data received from a not shown computer apparatusor the like, and outputs the data for every pixel as RGB data andattribute data indicating data attribute of each pixel. The attributedata holds attributes such as a character, a thin line, CG and a naturalimage. The image generation part 104 may be made up so as to process,other than image data received from the computer apparatus or the like,the image data from a reading component which is made up inside thecolor image forming apparatus. The reading component here includes a CCD(Charged Couple Device) or a CIS (Contact Image sensor) at least. Thereading component may be made up so as to include together a processingpart which carries out a prescribed image processing for the read imagedata. Without making it up inside the color image forming apparatus, itmay be made up so as to receive data from the reading component via anot shown interface.

Reference numeral 105 denotes a color conversion part and converts theRGB data into CMYK data corresponding to toner colors of the imageformation part 101, and stores the CMKY data and the attribute data in amemory part 106 which has a bit map memory. The memory part 106 is afirst memory part made up in the image processing part 102, and oncestores the raster image data for which the print processing is carriedout. The memory part 106 may be made up of a page memory which storesimage data for one page, and may be made up as a band memory whichmemorizes data for two or more lines.

Reference numerals 107C, 107M, 107Y and 107K denote HT (half toning)processing parts and carries out, for the attribute data and the data ofeach color outputted from the memory part 106, the interpolationprocessing which is the change less-than-one-pixel-basis, and carriesout the conversion of the tone of the input into pseudo half toneexpression by half toning processing. The interpolation processing inthe HT processing part 107 uses pixels before and after change pointscorresponding to the curvature characteristic which the image formingapparatus has. Details of the interpolation processing and the halftoning will be described later.

Reference numeral 108 denotes a second memory part included inside theimage forming apparatus, and stores N-value data processed by the HTprocessing part 107 (107C, 107M, 107Y and 107K). N indicates a numbersmaller than a tone number of the raster image data created by the imagegeneration part 104. In the case of a pixel position to beimage-processed after the memory part 108 being a change point, thechange for one pixel is carried out at the time of being read from thememory part 108. Details of the change for one pixel carried out in thememory part 108 will be described later. In the present embodiment,although each of a first memory part 106 and a second memory part 108was described as a separate constituent, a common memory part may bemade up inside the image forming apparatus.

FIG. 12A shows a figure illustrating schematically a state of data whichthe memory part 108 holds. As illustrated in FIG. 12A, in the statewhich the memory part 108 has stored, without depending on the changedirection as the image processing part 102, or on the curvaturecharacteristic of the image formation part 101, the data after theprocessing by the HT processing part 107 are held. At the time of the1201 line illustrated in FIG. 12A being read out, in the case of theprofile characteristic as the direction to be corrected in the imageprocessing part 102 being upward, the 1201 line is found to be in thestate where the line is displaced upward by one pixel with the changepoint as the boundary line as illustrated in FIG. 12B. In the case ofthe profile characteristic as the direction to be corrected in the imageprocessing part 102 being downward, the image data of the line 1201, atthe time of being read from the memory part 108, is found to be in thestate where the line is displaced downward by one pixel with the changepoint as the boundary line as illustrated in FIG. 12C.

Reference numeral 113 denotes PWM, and the image data for every colorwhich is read out from the memory part 108 after the change by one pixelbeing carried out, is converted into the exposure time of the scannerparts 114C, 114M, 114Y and 114K. Then, the image data after theconversion is outputted by a printing part 115 of the image formationpart 101.

The profile characteristic data which is already described asillustrated in FIG. 5C, inside the image formation part 101, asattributes which the image forming apparatus has, is held in an innerpart thereof. Then, the image processing part 102 is one where theprocessing is carried out according to the profile characteristic whichthe image formation part 101 holds (profiles 116C, 116M, 116Y and 116K).

Subsequently, using FIG. 6, operation of the HT processing part 107(107C, 107M, 107Y and 107K) of the image processing part 102 will bedescribed in detail. Since all of 107C, 107M, 107Y and 107K have thesame structure, they will be described as the HT processing part 107 inthe following.

The HT processing part 107 receives the image data of the correspondingcolor data from the CMYK data and the attribute data, and gives theimage data to screen processing parts 601 and 602.

The screen processing part 601, 602 receives the image data, and inorder to convert a continuous tone image into a halftone image havingfewer tone number, carries out the half toning by screen processing.

With reference to attribute information at this time, for example, foran image in which details of a line is considered to be important like acharacter, such screen processing as exceeds 200 lines and has higherresolution (high-number-of-lines screen processing part 602) is carriedout. On the contrary, in the case of the attribute where it is not thecase, such a screen (the low screen ruling processing part 601) as ishigher in tone, lower in resolution and less than 200 lines, is applied.After that, in a selector 603, with reference to the attribute, for aattribute portion of a character or a line, an output for which the highscreen ruling processing has been carried out is selected, and for theattribute which is not the case, an output for which the low screenruling processing has been carried out is selected, and the selectedoutput is outputted to an interpolation processing part 604. After that,for the image for which the screen processing has been carried out, theinterpolation processing is carried out in the interpolation processingpart 604, and level difference smoothing (interpolation processingless-than-one-pixel-basis) is carried out.

Using FIG. 7 and FIG. 8, a method of interpolating at the change pointwill be illustrated.

FIG. 8 shows an explanation of the interpolation processing which iscarried out conventionally, and note that the figure is used forcomparing with the present embodiment. FIG. 7 shows a figureillustrating the curvature characteristic of the image forming apparatusagainst the direction of laser scanning. A region 1, as the imageprocessing part 102, is the region where the correction must be carriedout upward, and on the contrary, a region 2, as the image processingpart 102, is the region where the correction must be carried outdownward. In the following description of the interpolation processing,as a matter of convenience for the description, the minimum intervalbetween two change points shall be 16 pixels, and however, the presentinvention is not limited to this. That is, it may be made anarbitrary-number-of-pixels interval, or a power-of-2 pixel interval forcircuitry reduction.

A pre-change-image before and after change point Pa in an example inFIG. 7, that is, an input-image-data configuration of the half toneprocessing part 107 is illustrated in (a) of FIG. 8.

An output-image-data configuration of the halftone processing part isillustrated in (c) of FIG. 8. In FIG. 8, a line of interest is thecenter line in the 3 lines of image data which are illustrated in thefigure. Change processing exceeding one pixel is carried out at thechange point at the time of being read from the memory part 108.Therefore, if processing filling up the gap is not carried out, a pixelconfiguration before and after the change point Pa will have caused alarge level difference corresponding to one pixel to appear with thechange point Pa as the boundary line.

If the simplest structure is considered in the case of carrying out thisinterpolation processing, by carrying out the following computation forthe line of interest and pixels located vertically in the subscanningdirection thereof, it can be realized.

A computing equation to be used for interpolation is given by formula 1

(Interpolation pixel values)=W1×(pixel values of 1 line preceding theline of interest)+W2×(pixel values of the line of interest)+W3×(pixelvalues of 1 line succeeding the line of interest)  (Formula 1)

* W1, W2, and W3: arbitrary weighting factors

The weighting in the present description, as illustrated in (b) of FIG.8, is described so that the total sum of 2 pixels in the subscanningdirection which become targets may become 16 in accordance with theminimum values of the change point. It is necessary to carried out theinterpolation so that, before and after the change point Pa in FIG. 7,the pixels of the line of interest may shift upward in the subscanningdirection. Accordingly, for the preceding line of the line of interest,the weighting value is set to 0,and for the line of interest and thesucceeding line of the line of interest, the weighting value is set upso that the pixel of the succeeding line may shift to the side of theline of interest gradually in (b) of FIG. 8. Carrying out processingwith the line of interest displaced allows the interpolation processingto be carried out so that pixels may shift upward in the subscanningdirection.

In the example of the present description, a conceptual diagram of theinterpolation pixel value given by the above formula 1 is illustrated in(c) of FIG. 8. By the interpolation by formula 1, before the changepoint Pa, the nearer the pixel is to the change point Pa, the more it isinfluenced by the pixel value of the succeeding line, and the furtherthe pixel is from the change point Pa, the more strongly it receives theinfluence of the line of interest, that is, a black data line.

As for the pixel after the change point Pa, it results that the nearerthe pixel is to the change point Pa, the more strongly it is influencedby the line of interest, and the further the pixel is from the changepoint Pa, the more strongly it is influenced by the succeeding line ofthe line of interest.

Finally, as illustrated in (d) of FIG. 8, in the memory part 108, thechange processing in one-pixel-basis is carried at the time ofoutputting of the memory part 108. Since the change processing exceedingone pixel is carried out at the time of being read from the memory part108, it results that the large level difference before and after thechange point Pa which had appeared so far disappears at this point.

As illustrated in (d) of FIG. 8, the line of 1 dot of the resolution(for example, 600 dpi) of the inputted image is converted (corrected indensity) into a half dot of 2 dots in the subscanning direction. Aspecific method of reproducing this half dot is the PWM illustrated by113C, 113M, 113Y and 113K, and is realized by modulating the laserexposure time. For example, in the case of the PWM having a 4-bit widthand 16 tones per one pixel, it controls the laser exposure time whiledividing the pulse width into sixteen for the tone 0 to 15, andswitching them gradually.

An aspect of the control of the laser exposure time is illustrated inFIG. 9. In FIG. 9, 16-division of the pulse of the exposure time for onepixel is carried out, and according to the pulse width of the16-division, the exposure time is controlled. In the figure, in the nextpulse, the aspect where 12 steps are made to be ON is illustrated, andin the next pulse thereof, 8 steps are made to be ON. Although, in thefigure, a theoretical rectangular waveform is illustrated, in a actualelectric signal, the waveform becomes blunt, and at the rising andfalling edge part, the exposure equivalent to ON may not always be ableto be carried out.

In the following description, the pixel where the exposure by the PWM iscarried out during all over the time of 16/16 is called a full dot, andthe pixel where the exposure is carried out during the time of 1/16 to15/16 is called a half dot.

In FIGS. 10A to 10C, an aspect of centroid movement using the half dotby laser power modulation is illustrated. This figure illustrates theaspect where the centroid is displaced in an order of FIG. 10A, FIG. 10Band FIG. 10C, from the right scanning line to the left scanning linegradually. A curved line indicated with a dotted line indicates anexposure image formed by carrying out one laser scan, and a curved lineindicated with a continuous line indicates the exposure image includingalso an influence of an adjoining laser exposure. The centroid movementin the interpolation processing is carried out based on deviation amountfrom a laser scanning position. It seems that the centroid movement tothe left occurs surely with an integral value kept, and however, angenerated shape is not always in agreement, and it may appear as avariation of the density. Therefore, even if density preservation iscarried out from the view point of a signal value or an amount ofintegrated light, an outputted image may not maintain the density. Thatis, against an exposure amount or the exposure time, a situation whereit is hard to make the linearity of the density come out may arise.

That is, this means that, even if what is exposed during the time of6/16 adjoins what is exposed during the time of 10/16, it is difficultto always realize the density substantially equal to that in theexposure by 16/16. Even by a size of an adjoining dot, it may bedifficult to express the linearity. For example, even though, in thecase of a line having sufficiently thick 10-dot width, it is possible tomaintain line width uniformity with the level difference canceled by theinterpolation pixel value given by the above formula, in the case ofinterpolating a line having 1-dot width by the same formula, a thicknessof the line will have changed depending on the weighting factoraccording to density variation. By adjusting these individually, methodof calculating a PWM amount to be outputted linearly, method ofincreasing a number of divisions of the PWM from the above-mentioned 16in order to increase the tone number, and others are considered.However, because, depending on an environment, a state of the developingdevice or a durability state of a printer, a method of adjustment alsovaries, it is difficult to guarantee the linearity.

That is, in the actual condition, rather than processing guaranteeingthe linearity itself against the density, in the state using the PWMwhere it is hard for the linearity of the density to come out, needed isa method of maintaining density uniformity with the level differencebefore and after the change point of one dot small.

Although it is hard for the linearity here to come out, as aprerequisite, the following things can be said. That is, although thelinearity of the density is not guaranteed against the pulse width of 0to 16, the pulse width of 2 rather than 0, 3 rather than 2, or 16 ratherthan 15, will allow a thick dot to be formed. Also with respect toadjoining of dots, if it is hard for the linearity to come out, forexample, the density at the time when two dots each having a pulse widthof 13 adjoin each other in subscanning direction, and the density at thetime when a dot having a pulse width of 16 and a dot having a pulsewidth of 10 adjoin each other in the subscanning direction where the sumtotal value is the same as 26, may be different from each other.However, even if it is said that it is hard to make the linearity comeout, the density of 2 dots where the dot of the pulse width of 10 islocated right above the dot having the pulse width of 16 in thesubscanning direction, and the density of 2 dots where the dot of thepulse width of 10 is located right below the dot having the pulse widthof 16 in the subscanning direction, are substantially equivalent.

Then, in the present embodiment, combinations of half dots are minimizedin order to maintain the density uniformity. Accordingly, against aninput as illustrated in (a) of FIG. 11, the weighting factor W indicatedin (b) of FIG. 11 are set up. FIG. 11 shows a figure compared with aconventional example of FIG. 8, and Pa and Pb in the figure, asillustrated in FIG. 7, indicates the change point where the change inone-pixel-basis is carried out upward in the subscanning direction (atthe side of preceding line). A conceptual diagram of an output acquiredby setting up the weighting factor W as illustrated in (b) of FIG. 11becomes as illustrated in (c) of FIG. 11. In this case, an input of onedot is always expanded into 2 dots in the subscanning direction, andmoreover, the vertical ratio is limited to 2 ways of 1:3 or 3:1.

Therefore, it results that, as for the line of one dot which isillustrated in the figure, a dot having the exposure time of 4/16, and adot having the exposure time of 12/16 are made to perform verticalinversion at the change point and the center point of the change points(Pa′, Pb′ in the figure). Therefore, between the two factors, only bythe way of locating dots vertically, the density based on half dotsbecomes equivalent, and the level difference is smoothed into the linehaving 1-or-less dot.

In FIG. 13, details are described with respect to the state before andafter the change point Pa. Against an input of the (a) in FIG. 13, thecentroid is displaced downward by ¼ pixel in the (b), similarly, thecentroid is displaced upward by ¼ pixel in the (c). That is, a distancebetween centroids (b) and (c) corresponds to ½ pixel, and the leveldifference which sandwiches the change point Pa is smoothed into thelevel difference of ½ pixel.

Finally, as illustrated in (d) of FIG. 11, in the memory part 108, thechange processing in one-pixel-basis is carried out at the time ofoutputting from the memory part 108. In the same way as the previousdescription, since the change processing exceeding one pixel is carriedout at the time of being read out from the memory part 108, it resultsthat the large level difference before and after the change point Padisappears at this point.

For example, in the example described in FIG. 11, it is possible that,in the case of a laser scanning resolution of 600 dpi, the leveldifference which occurs at the change point is made small to anequivalent 1200 dpi as the half thereof (smoothed into the leveldifference of ½ pixel).

Since, as for the PWM tone as mentioned above, it is hard for thelinearity of the density to come out, it is considered thoroughly thatthe centroid movement by ¼ pixel have not been achieved even by creating2 dots each having a ratio of 4:12 and paying attention only to 1 dotpart thereof. In that case, unless a relation of the density to theexposure time is inverted, it is guaranteed at least that the centroidmovement has been settled in the level difference which is smaller thanone pixel. When it is hard for the linearity of the density to come outin the case of using the PWM tone, the most suitable centroid movementratio may not always be 1:3 (4:12) as mentioned above. In combinationwith the tone of the PWM, it is also possible to displace the ratio like3:13, and use a combination where the sum total thereof does not become16 as 4:11.

In this way, in the present embodiment, it is characterized in that 1dot is converted into 2 dots each having a half dot, where the way oflocating the half dots maintain the relation of the vertical inversionin the subscanning direction, and the centroid movement less than 1 dotis made to be carried out. Thereby, even when it is hard for thelinearity to come out in the relation between the PWM tone and thedensity, it is possible that the level difference is made into less than1 dot without causing unevenness in the line width and density.

It is described here, as a example, that the image data is correctedupward, and however, it is also possible as a matter of course that,while the way of locating the half dot is made to be inverted in thesubscanning direction and the density is made to be equivalent, theimage data is corrected downward.

Embodiment 2

In the embodiment 1 above, described was an example where theinterpolation processing, to the line or the full dot having one dot,gives half dots maintaining the relation of vertical inversion so thatunevenness in density or thickness changes may not be caused, carriesout the centroid movement less than one dot, and smoothes the leveldifference. In the case of the inputted image on which carried out wasprocessed by the one bit binary halftone of all-exposure-ON andall-exposure-OFF (2 ways of 16/16 and 0/16) by using the PWM of 16steps, it is possible to carry out the centroid movement as theabove-mentioned embodiment 1.

That is, the method of Embodiment 1 is effective for a one bit, that is,a binary image for which even in the case of the tone number of theinput being, for example, 256 tones per pixel, etc., halftone processingis always carried out, and conversion thereof into ON or OFF is alwayscarried out. However, the inputted image may include, other than theabove full dot image, one which is N-value-ized, specificallysexadecimal-ized by 4 bits including many of a half dot for example, inthe halftone processing. In such a case, it is difficult to carry outthe centroid movement simply.

In Embodiment 2, with 4-bit halftone screen processing as an example,described is a configuration in which, for a dot expressing the 4-bithalftone, the way of locating the half dot maintains the relation of thevertical inversion with the change point as a border, and the densityequivalence is realized.

In the present embodiment, details are described with respect to the HTprocessing part illustrated in 107, and the description before and afterthe processing is the same as Embodiment 1, and therefore, is omitted.

The screen processing in the HT processing part 107 described also inEmbodiment 1 is based on a dither method where arbitrary thresholdvalues are read from a dither matrix with a plurality of thresholdvalues arranged, and inputted image data is N-value-ized by beingcompared with the threshold values.

Using FIG. 14, with respect to the dither method, a detailed principlewill be described. The continuous tone image of the input (for example,an 8-bit 256 tone image) is divided into a block of N×M (8×8 in thefigure). After that, a magnitude comparison of the tone value of everypixel in the block is carried out with each threshold value in thedither matrix in which threshold values have been arranged in same N×Msize as the block, and for example, 1 will be outputted if a pixel valueis larger than a threshold value, and 0 will be outputted if it is notmore than the threshold value. By carrying out this against all thepixels for every size of the matrix, it becomes possible to carry outbinarization of the overall image. In the electrophotographic colorimage forming apparatus, in order to realize stable dot reproducibilityon a recording medium, the dither matrix where dots are concentrated isperiodically used. On the contrary, if dots have been diffused orisolated dots each of which has no dots in the periphery thereofincreases, a stable dot reproducibility is not acquired. In the case ofthe high screen ruling described in the above embodiment 1, the dotspace is narrow, and on the contrary, in the case of the low screenruling, the dot space becomes wide.

A schematic diagram of tone change using a dither matrix is illustratedin FIGS. 15A and 15B. A continuous tone image as illustrated in FIG. 15Ais expressed as a binary image as illustrated in FIG. 15B.

By expanding this threshold value matrix to 15 sheets, and preparing athreshold value of one tone per a sheet, an output of a16-tone-4-bits-screen becomes possible.

Usually, in a cycle of the screen, certain one dot begins to light upfrom a low density toward a high density, and after the dot has reacheda full dot, dots in the circumference of the full dot begins to lightup. That is, the 2nd and subsequent dot after the full dot will alwaysadjoin the full dot. In this way, by lighting up dots with themcentralized, a stable dot formation can be realized, and the more sparsedots are, the fewer isolated dots become, and it is possible toexpresses the tone in a stable state. The screen is formed in the orderof the lighting of dots to express the intermediate density.

If for such a screen image, multiplying by weighting factors andcomputing are carried out using a method indicated in Embodiment 1, aregion of half dots is diffused vertically, and with a change pointsandwiched, a texture of the screen is deformed and a tone gap will havebeen caused.

In FIG. 16, an image after carrying out the screen processing, andagainst it, an image in which weighting factors with the change point Paas a border are switched, as illustrated in FIG. 11, are illustrated. Animage illustrated in (a) of FIG. 16 becomes an example of an image afterthe 16-tone-4-bits-screen mentioned above. An image in which, for thisimage, weighting computation described in Embodiment 1 is carried out,and the centroid is displaced downward by ¼ pixel, will become (b).Similarly, an image in which the centroid is displaced upward by ¼pixel, will become (c). (d) of FIG. 16 illustrates a sample taken outfrom a part of pixels of (a) of FIG. 16, and the PWM value thereof.Similarly, (e) of FIG. 16 illustrates a sample taken out from a part ofpixels of the same position of (b) of FIG. 16 as (d) of FIG. 16, and (f)of FIG. 16 illustrates a sample taken out from a part of pixels of thesame position of (c) of FIG. 16 as (d) of FIG. 16. In this way, it seemsthat, by carrying out the weighting computation even for an image afterthe 16-tone-4-bits-screen, the centroid moves on a digital data, and thedensity of the sum total is also held.

However, in a state where it is hard for the linearity against the PWMto come out easily, that is, in a state where, twice the density has notcome out in 4 for 2, or the density equivalent to 9 does not come outeven if dots of 4 and 5 adjoins each other, the way of locating a halfdot for the dot of the screen will have changed with the change point asa border. As a result, on the image outputted, the density variation iscaused with the change point as a border, and will have become apparentas inconsistencies.

Although, before the interpolation processing, the region was expandedwith dots centralized, the dots are diffused under the influence of theinterpolation, and the region of half dots will have expanded. In thecase of FIG. 16, although in (d), two full dots and one half dotexisted, in (e) and (f), it has been expanded to one full dot and threehalf dots. In this way, when the weighting computation described inEmbodiment 1 is carried out for an image where the halftone processingof a multiple-value tone is carried out, and the interpolationprocessing is carried out, it becomes, as a result, difficult to createa stable dot, and the stability of half tone density will also have beenimpaired.

Then, in order that the density of a dot is not made to be changed withthe change point as a border, a method of displacing the centroid withthe way of locating a half dot or a degree of concentration of dotsmaintained will be needed.

Then, in the present embodiment, a method to maintain a verticalsymmetrical figure of dots which appears on both sides with the changepoint as a border by exerting ingenuity for the screen processing, willbe described using FIG. 17.

In a halftone processing part 601, first in a subscanning-resolution-4times-conversion-part 1701, the inputted multi tone raster image data ofeach CMYK color of a mainscanning 600 dpi and a subscanning 600 dpi isonce made to be expanded to 4 times the resolution in the subscanningdirection. This method is carried out by simple padding of a repetitionof data. Thus, after preparing the image of a mainscanning 600 dpi and asubscanning 2400 dpi, in a screen processing part 1702, a screenprocessing having 5 tones of 0 to 4 is applied to this image which hasbeen converted into 4 times the resolution in the subscanning direction.That is, as a result of the screen processing, an image having the toneof values of 0 to 4 which has been expanded to 4 times the resolution inthe subscanning direction is outputted.

After that, in an interpolation processing part 604, in asubscanning-position-displacing-sampling part 1703, a tone value for 4dots is added in the subscanning direction, and it is converted into anew dot, and sampling is going to be carried out toward the size of ¼ inthe subscanning direction, that is, the subscanning 600 dpi of theoriginal resolution. That is, in the processing illustrated in FIG. 17,the screen processing is carried out in the state where resolution hasbeen expanded, and processing which returns the image with the screenprocessing finished to the image of the original resolution is carriedout. The interpolation processing part 604, in the case of carrying outsampling toward the original resolution, carries out the interpolationprocessing by displacing a position where the sampling is carried out inconsideration of the deviation amount described in FIG. 7, and is goingto output the PWM values of 0 to 16. Then, as described in Embodiment 1,by controlling the position for 4 dots which is added in thesubscanning-position-displacing-sampling part 1703 so that the verticalinversion may occur with the change point as a border, the interpolationand the smoothing are realized. As for the output of the interpolationprocessing part, the raster image data of the mainscanning 600 dpi andthe subscanning 600 dpi, having the PWM value of 0 to 16 per one pixelof each CMYK color is outputted. Because 5 bits are needed as the datalength if outputted as it is, the value is thinned out and rounded offto the value of 0 to 15.

Here, 0 to 15 are outputted as it is, for example, and in the case of16, that is, full pixel ON, it will be rounded off to 15.

FIG. 18 shows a figure illustrating schematically processing accordingto the change of a pixel in Embodiment 2. (a) of FIG. 18 illustrates anaspect where an image expanded to 4 times the resolution in thesubscanning direction is inputted. A region surrounded by a wide line in(a) of FIG. 18 corresponds to the region for one dot of the originalresolution. (b) of FIG. 18 illustrates a phase in which the tone valuefor 4 dots is added and the sampling is carried out into the new one dotfor an input as illustrated in (a) of FIG. 18. (b) of FIG. 18illustrates an aspect where the phase is shifted with the change pointPa, Pb and Pa′, Pb′ located at the middle position thereof as a border.A conceptual diagram of an output acquired by sampling with the phaseshifted like this becomes as illustrated in (c) of FIG. 18. In (b) FIG.18, for the region of Pa′ to Pa, a sampling for 4 dots is carried out atthe position displaced by ¼ pixel in the preceding line direction. Forthe region of Pa to Pb′, a sampling for 4 dots is carried out at theposition displaced by ¼ pixel in the succeeding line direction. Withrespect to the sampling position (that is, shift of the phase) of a dot,it will be computed depending on the amount of the registrationdeviation (profile characteristic) as described in FIG. 7.

In the case of (c) of FIG. 18, an input of one dot is always expandedinto 2 dots in the subscanning direction, and moreover, the verticalratio is limited to 2 ways of 1:3 or 3:1, and the same result as thecase described in Embodiment 1 has been acquired.

Therefore, it results that, as for the line of one dot which isillustrated in the figure, a dot having the exposure time of 4/16 and adot having the exposure time of 12/16 are made to perform verticalinversion at the center point of the change points (Pa′, Pb′ in thefigure). Therefore, between the two regions, only by the way of locatingdots changing vertically, the density based on half dots becomesequivalent, and the level difference is smoothed into the line having1-or-less dot.

Finally, as illustrated in (d) of FIG. 18, in the memory part 108, thechange processing in one-pixel-basis is carried out at the time ofoutputting from the memory part 108. In the same way as above, since thechange processing exceeding one pixel is carried out at the time ofbeing read out from the memory part 108, it results that the large leveldifference before and after the change point Pa will disappear here.

Next, not the line image of a full dot as illustrated in FIG. 18, but anexample in the case of a halftone image being inputted after screenprocessing including a half dot will be described using FIG. 19. (a) ofFIG. 19 illustrates an image of a mainscanning 600 dpi and ansubscanning 2400 dpi, after a screen processing of 4 tones for each dot.Because, in this figure, the resolution differs by the mainscanning andthe subscanning, and is coarse in the mainscanning, a pixel is expressedin a landscape shape. A square grid on this image becomes the grid ofthe subscanning 600 dpi. Against this image, an image which is sampledinto the mainscanning 600 dpi and subscanning 600 dpi after displacingthe centroid downward by ¼ pixel in terms of 600 dpi is (b). Similarly,an image in which the centroid is displaced upward by ¼ pixel and thesampling is carried out is (c).

In (d)of FIG. 19, as a sample which was taken out from a part of (a),illustrated is a dot sequence after the screen of 20 pixels in terms of2400 dpi and 5 pixels in terms of 600 dpi in the subscanning directionat a certain main scanning position. As illustrated in this (d), theorder of the screen is defined so that dots may be going to light up ina vertical symmetry. Thereby, after that, even if the centroid isdisplaced downward by ¼ pixel and the sampling is carried out like adotted line of (d), the total value as a matter of course, the shape ofthe dot which only carries out vertical inversion, and also the way oflocating half dots, becomes the same, like (e). It becomes possible toalso minimize diffusion of dots. Similarly, even if the centroid isdisplaced upward by ¼ pixel and the sampling is carried out like adotted line of (f), the total value as a matter of course, the shape ofthe dot which only carries out vertical inversion, and also the way oflocating half dots, becomes the same, like (g). It becomes possible toalso minimize diffusion of dots.

In this way, in the present embodiment, by the simple processing thatthe screen processing is carried out in a state where the resolution isexpanded in the subscanning direction, and an image with the screenfinished is returned to the original resolution in consideration of thedeviation amount, it becomes possible to carry out the centroid movementless than one dot. In particular, also for an image after the screenprocessing including half dots, it becomes possible to carry out thecentroid movement less than one dot as the way of locating the half dotmaintains the relation of vertical inversion in the subscanningdirection. Thereby, even when it is hard for the linearity to come outin the relation between the PWM tone and the density, it becomespossible to make the level difference into less than one dot withoutcausing unevenness in density against the image after the screen.

Here, as a matter of convenience for the description, the PWM wasdescribed by 16 divisions as an example where it is hard for linearityof the density to come out, but it is not limited to this.

In the description in this time, an example in which the resolution wasincreased by 4 times only in the subscanning direction, and was made tobe 2400 dpi-nized was described, and however, even by carrying out ascreen processing of a 1-bit-binary after making it to be 2400 dpiincluding also the mainscanning, the same as above is possible. Althoughthe resolution was also described using the input of 600 dpi, it is notlimited to this.

Embodiment 3

In the above Embodiment 2, by increasing the resolution in thesubscanning direction before the screen processing, in order that theunevenness in density or the thickness changes may not be caused by theinterpolation processing, half dots which maintains the relation ofvertical inversion has been given, the centroid movement less than onedot has been carried out, and the level difference has been smoothed.

In Embodiment 3, described is a configuration in which, withoutincreasing the resolution before the screen processing, for a dotexpressing the 4-bit halftone, the way of locating the half dotmaintains the relation of the vertical inversion with the change pointas a border, and the density equivalence is realized.

In the present embodiment, details are described with respect to the HTprocessing part illustrated in 107, and the description before and afterthe processing is the same as Embodiment 1, and therefore, is omitted.

Detailed explanation is carried out using FIG. 20. In a screenprocessing part 2001, for the inputted multi tone raster image data ofeach CMYK color of a mainscanning 600 dpi and a subscanning 600 dpi, ascreen processing which has 16 tones of 0 to 15 is applied for theimage. For the data, flag information of 3 values indicating a directionof up-down-center in which a dot gets up is given for each pixel in anup-down-center flag-giving part. This flag is defined, for example, ascenter for 0, down for 1 and up for 2. This flag defines the directionin which a dot gets up to the dot which begins to light up first as adot of the screen.

After that, in the interpolation processing part 604, in a subscanningexpanding part 2003 based on up-down and getting-up, in consideration ofgetting-up-locating thereof based on 4-bit image data and 3-value flaginformation, 4-bit image data are expanded 4 times in the subscanningdirection. The details are illustrated in FIG. 21. As illustrated in thefigure, even in the case of the pixel value having the same value thatis 10, the getting-up-locating of the data that is expanded depending onthe flag information differs by the up-down-center. If it is (a),because a flag of 0 which means the center is given, as for 4 pixels ofthe output, 4 is outputted to 2 pixels in the center, and 1 is outputtedto the up side and down side thereof. If it is (b), because a flag of 1which means the down side is given, each of 2 pixels of the down sidebecomes 4,and the up side pixel thereof becomes 2, and the top becomes0, and in the case of a flag of 2 which means the up side being given asillustrated in (c), it becomes a reverse case thereof.

After that, as described also in Embodiment 2, the tone value for 4 dotsis added in the subscanning direction, and it is converted into a newdot, and sampling will be carried out toward the size of ¼ in thesubscanning direction. As for a value outputted, as the PWM value of 0to 16, the interpolation part is going to output it.

An illustrative example, with the FIG. 19 as an exemplar, is illustratedin FIG. 22. When an image after the screen of 600 dpi as illustrated in(a) of FIG. 22, and flags as illustrated in (b) by a flag giving part,are inputted into the subscanning expanding part 2003 based on up-downand getting-up, the output thereof becomes a shape where images get uptoward the center as shown in (c).

Processing in the following is the same as that of Embodiment 2. Asillustrated in this (a) of FIG. 22, the order of the screen is definedso that dots may be going to light up in a vertical symmetry, and flagsare given as illustrated in (b). Thereby, after that, even if thesampling is carried out like (d) and (e), or even if the sampling iscarried out like (f) and (g), the total value as a matter of course, theshape of the dot which only carries out vertical inversion, and also theway of locating half dots, become the same, and it becomes possible toalso minimize diffusion of dots.

The flag indicating a direction in which a dot gets up is defined as amatrix together with the threshold value matrix of the screen, and isset in the up-down-center flag-giving part. The matrix is realizable bydefining a dot which lights up first in isolation in the subscanningdirection so as to be located at the center, and defining a dot whichlights up after that is so as to get up to the dot which has beenlighted up already. This order of lighting up can be easily preparedfrom the threshold value matrix of the screen.

Thereby, it becomes possible to make the output of the halftoneprocessing part be the image of a 4-bit 600 dpi and the flag data of 2bits of resolution thereof, and against the data of 2400 dpi in theabove Embodiment 2, an equivalent effect can be realized by small data.

Other Embodiments

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment (s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment (s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-260320, filed Nov. 13, 2009, which is hereby incorporated byreference herein in its entirety.

1. An image forming apparatus comprising: an interpolation processingcomponent configured to carry out less-than-one-pixel-basis correctionof image data by correcting density of each pixel of the image data sothat sequence of pulse width according to the density of the pixel in asubscanning direction comes into inverted relationship before and aftera change point that is a location where one-pixel-basis correction iscarried out for the image data; and an image forming componentconfigured to form an image based on a signal acquired by carrying out apulse width modulation according to the corrected density of the imagedata.
 2. The image forming apparatus according to claim 1 furthercomprising a half toning component configured to convert inputted imagedata of M-value into the image data of N-value (M>N) using a dithermethod, and output the converted image data to the interpolationprocessing component.
 3. The image forming apparatus according to claim2 wherein the half toning component uses the dither method in whichorder of lighting of dots lights up in a vertical symmetry in thesubscanning direction.
 4. The image forming apparatus according to claim2, wherein the half toning component converts inputted image data ofM-value into the image data having high resolution at least in thesubscanning direction, and converts the converted image data into theimage data of N-value using the dither method, and the interpolationprocessing component carries out the less-than-one-pixel-basiscorrection by carrying out conversion into the image data having lowerresolution than the high resolution by shifting a phase.
 5. The imageforming apparatus according to claim 3, wherein the half toningcomponent converts inputted image data of M-value into the image datahaving high resolution at least in the subscanning direction, andconverts the converted image data into the image data of N-value usingthe dither method, and the interpolation processing component carriesout the less-than-one-pixel-basis correction by carrying out conversioninto the image data having lower resolution than the high resolution byshifting a phase.
 6. The image forming apparatus according to claim 2,wherein the half toning component further outputs a flag indicating adeviation amount related to the image data in the subscanning directionto the interpolation processing component, and the interpolationprocessing component carries out the less-than-one-pixel-basiscorrection by converting the image data based on the flag.
 7. The imageforming apparatus according to claim 3, wherein the half toningcomponent further outputs a flag indicating a deviation amount relatedto the image data in the subscanning direction to the interpolationprocessing component, and the interpolation processing component carriesout the less-than-one-pixel-basis correction by converting the imagedata based on the flag.
 8. An image forming method, comprising the stepsof: carrying out interpolation processing of carrying outless-than-one-pixel-basis correction of image data by correcting densityof each pixel of the image data so that sequence of pulse widthaccording to the density of the pixel in a subscanning direction comesinto inverted relationship before and after a change point that is alocation where one-pixel-basis correction is carried out for the imagedata; and carrying out image formation of forming an image based on asignal acquired by carrying out a pulse width modulation according tothe corrected density of the image data.
 9. A computer-readablerecording medium with a program making a computer execute an imageforming method recorded therein, the image forming method comprising thesteps of: carrying out interpolation processing of carrying outless-than-one-pixel-basis correction of image data by correcting densityof each pixel of the image data so that sequence of pulse widthaccording to the density of the pixel in a subscanning direction comesinto inverted relationship before and after a change point that is alocation where one-pixel-basis correction is carried out for the imagedata; and carrying out image formation of forming an image based on asignal acquired by carrying out a pulse width modulation according tothe corrected density of the image data.